powerpc/64s/hash: Restrict stress_hpt_struct memblock region to within RMA limit

powerpc/64s/hash: Restrict stress_hpt_struct memblock region to within RMA limit

When HV=0 & IR/DR=0, the Hash MMU is said to be in Virtual Real
Addressing Mode during early boot. During this, we should ensure that
memory region allocations for stress_hpt_struct should happen from
within RMA region as otherwise the boot might get stuck while doing
memset of this region.

History behind why do we have RMA region limitation is better explained
in these 2 patches [1] & [2]. This patch ensures that memset to
stress_hpt_struct during early boot does not cross ppc64_rma_size
boundary.

[1]: https://lore.kernel.org/all/20190710052018.14628-1-sjitindarsingh@gmail.com/
[2]: https://lore.kernel.org/all/87wp54usvj.fsf@linux.vnet.ibm.com/

Fixes: 6b34a099faa12 ("powerpc/64s/hash: add stress_hpt kernel boot option to increase hash faults")
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/ada1173933ea7617a994d6ee3e54ced8797339fc.1761834163.git.ritesh.list@gmail.com